1. Field of the Invention
The invention relates to optical disk drives and method, and more particularly to optical disk drives and method for controlling track seeking for optical disk drives.
2. Description of the Related Art
Data is recorded on different positions of an optical disk. When an optical disk drive wants to read specific data from an optical disk, the optical disk drive must project a laserbeam of a pickup head on a target position corresponding to an address of the specific data in advance, and then derive data from reflection of the laserbeam. The position of a spot of the laserbeam projected on the optical disk surface is determined by a position of the pickup head and a position of an objective lens in the pickup head. The optical disk drive therefore must adjust the positions of both the objective lens and the pickup head to aim at the spot of the laserbeam at the target position of the specific data.
An optical disk drive performs a track-seeking procedure to move a spot of a laserbeam emitted by a pickup head from an original position to a target position on a disk surface. A position of a pickup head is controlled by a sled, and a position of an objective lens in the pickup head is controlled by a tracking coil. The track-seeking procedure therefore comprises adjusting the position of the pickup head by the sled and adjusting the position of the objective lens by the tracking coil.
Referring to FIG. 1, a block diagram of a conventional optical disk drive 100 is shown. The optical disk drive 100 comprises a pickup head 102, an equalizer 104, a demodulator 106, a decoder 108, a seek control device 112, and a driver IC 114. The pickup head 102 comprises an objective lens 118 projecting a laser beam on the surface of a disk 120 and a sled 116 adjusting the position of the pickup head 102. When the optical disk drive 100 requires reading data stored on a specific portion of the disk 120, the seek control device 112 first generates a tracking control output (TRO) signal for controlling a tracking coil in the pickup head 102 and a feed motor output (FMO) signal for controlling a stepping motor (not shown). The driver IC 114 then derives FMO1 and FMO2 signals from the FMO signal, and the stepping motor adjusts a position of the sled 116 according to the FMO1 and FMO2 signals. Similarly, the driver IC 114 derives TR+ and TR− signals from the TRO signal for the tracking coil to adjust a position of the objective lens 118 on the sled 116.
After both the positions of the sled 116 and the objective lens 118 are shifted, the laserbeam projected by the lens 118 is aimed at the specific portion of the disk 120 storing the specific data and reflection of the laserbeam is generated. The equalizer 104 then derives a radio frequency (RF) signal from the reflection of the laserbeam. The demodulator 106 and the decoder 108 then respectively demodulate and decode the RF signal to obtain the specific data and address thereof. The equalizer 104 also derives servo signals such as a tracking error (TE) signal and a focusing error (FE) signal from the reflection. The seek control device 112 can therefore generates the TRO signal and the FMO signal according to the servo signals to control the position of the lens 118 and the sled 116.
The position of lens 118 in the pickup head 102 is defined with a shift distance counted from an origin at a middle point of the sled 116. When the lens 118 is shifted with different shift distances in the pickup head 102, the strength of the reflection from the disk 120 also differs, affecting quality of servo signals derived from the reflection. Referring to FIG. 2, shows an example of a relation between amplitude of a servo signal and different shift distances of the objective lens 118, which is also known as the vision characteristic of the pickup head 102. When the objective lens 118 is located at the origin, the shift distance is 0, and the largest amplitude of servo signal is around the origin point. When the shift distance is positive and increases, the objective lens 118 is shifted towards one side of the sled 116, and the amplitude of the servo signal slightly decreases. When the shift distance is negative and decreases, the objective lens 118 is shifted towards the other side of the sled 116, and the amplitude of the servo signal significantly decreases. The objective lens 118 should be prevented from being moved to positions with large negative shift distances, since the objective lens 118 with a positive shift distance generates a servo signal with larger amplitude. For example, if the objective lens 118 is shifted to a position with a shift distance of −0.5 mm, the amplitude of the servo signal is lowered to almost a half of the amplitude of the servo signal generated by a lens at the origin, and the low amplitude of the servo signal may cause errors in operation of the optical disk drive.
The conventional optical disk drive 100 therefore may erroneously operate according to servo signals with low amplitudes due to the objective lens 118 with a negative shift distance. In addition, an eccentric disk 120 further deteriorates the problem. Referring to FIG. 3, a schematic diagram of oscillation of a lens in correspondence with oscillation of a disk is shown. If the optical disk 120 is an eccentric disk, a track of the eccentric disk 120 swings back and forth when the disk 120 is rotated, as shown by the disks 310 and 320. The objective lens 118 must therefore oscillate back and forth to fix a spot of the laserbeam on the track, as shown by the lens 330 and 340. When a bias distance of an eccentric track is 0.28 μm, the oscillation distance of the track is 0.56 μm, and the objective lens 118 must be accordingly shifted with an oscillation distance d of 0.56 μm. If the objective lens 118 is originally located at a position with a negative shift distance, the oscillation due to eccentric disk may shift the objective lens 118 to a position with a large negative shift distance, and the amplitude of the servo signal generated therefrom will greatly decrease as shown in FIG. 2. An optical disk drive therefore requires a mechanism to prevent the lens from an extreme shift distance when the optical disk drive adjusts the position of the lens for track-seeking.